Arthropods are characterised by a rigid, articulating, exoskeleton operated by a lever-like system of segmentallyarranged, antagonistic muscles. This skeletomuscular system evolved from anunsegmented body wall musculature acting on a hydrostatic skeleton, similar tothat of the arthropods’ close relatives, the soft-bodied onychophorans. Unfortunately,fossil evidence documenting this transition is scarce. Exceptionally‑preservedpanarthropods from the Cambrian Lagerstätte of Sirius Passet, Greenland,including the soft-bodied stem-arthropod,Pambdelurion whittingtoni, and the hard-bodied arthropods, Kiisortoqia soperi and Campanamuta mantonae, are unique in preservingextensive musculature.Here we show that Pambdelurion’s myoanatomyconforms closely to that of extant onychophorans, with unsegmented dorsal,ventral and longitudinal muscle groups in the trunk, and extrinsic and intrinsicmuscles controlling the legs. Pambdelurionalso possesses oblique musculature, which has previously been interpretedas an arthropodan characteristic. However, this oblique musculature appears confinedto the cephalic region and first few body segments and does not represent ashift towards arthropodan myoanatomy. The Sirius Passet arthropods, Kiisortoqia and Campanamuta, also possess large longitudinal muscles in the trunk,although, unlike Pambdelurion, theyare segmentally divided at the tergal boundaries. Thus, the transition towardsan arthropodan myoanatomy from a lobopodian ancestor likely involved thedivision of the peripheral longitudinal muscle into segmented units.
- Sirius Passet